JP2011-241947A discloses a hybrid construction machine capable of adding a discharge pressure of an assist pump driven by a motor to discharge pressures of main pumps driven by an engine. The hybrid construction machine includes a first and a second variable-capacity type main pump.
The first main pump is connected to a first circuit system by way of a first supply passage and a plurality of operation valves are connected to the first circuit system. An output port of a first logic valve is connected to the first supply passage. An input port of the first logic valve constantly communicates with the assist pump of the variable-capacity type by way of a joint passage.
The second main pump is connected to a second circuit system by way of a second supply passage and a plurality of operation valves are connected to the second circuit system. A second logic valve is disposed in the second supply passage. An input port of the second logic valve is connected to the second main pump via a second supply passage on the upstream side of the second logic valve. An output port of the second logic valve is connected to the second circuit system via the second supply passage on the downstream side of the second logic valve.
The assist pump of the variable-capacity type integrally rotates in coordination with a hydraulic motor and a motor generator of the variable-capacity type. The motor generator is connected to a battery via an inverter. Thus, if the hydraulic motor rotates, the motor generator rotates to generate power and the generated power is stored into the battery via the inverter.
A switching valve is connected to the second supply passage. The switching valve is normally kept at a neutral position by the action of a centering spring and allows the joint passage communicating with the assist pump to communicate with the second supply passage by way of a branch passage. A check valve for permitting only a flow from the switching valve to the second supply passage is provided in the branch passage.
Accordingly, when the switching valve is at the neutral position, the first and second logic valves are connected in parallel to the joint passage.